The invention relates to a speed control circuit for a single phase AC induction motor.
In single phase AC induction motors, it is known in the prior art to reduce the terminal voltage while at line frequency to control motor speed. Various methods are known in the prior art for varying the voltage, including the use of rheostats, variable voltage transformers, and thyristor phase controls. Reducing the terminal voltage reduces the flux in the motor, therefore, the motor must slip more to produce the rotor current necessary to maintain torque and the motor slows down. However, the torque produced is the product of the flux and the rotor current. So if the torque required by the load is not also decreasing, then the motor's internal impedance will not support the rotor current and the motor stops.
Starting a stopped motor requires more voltage than running a motor. This poses a practical limit on the speed range of this type of motor speed controller.
A triac phase control system is known in the prior art and is often applied to single phase shaded pole and permanent split capacitor motors driving fans, blowers and centrifugal pumps. The triac phase control typically has a two to one voltage range and about the same speed range. The motor generally won't start with less than half the rated voltage.
A triac phase control controls the phase angle during the half cycle of the AC power supply at which the triac turns on. A capacitor is charged through one or more rheostats to a given threshold voltage which triggers the triac. Reducing the resistance of the rheostats increases the current into the capacitor, which decreases the time delay interval in firing the triac, and increases the voltage across the motor.
The present invention overcomes the starting and speed range limitations in the prior art. The invention provides an additional boost circuit for the triac phase control and starts the motor with almost full voltage and subsequently reduces the voltage as the motor begins running.